Hi all,
The need for a simple amp has become to much for me, so I decided to go for the JE Labs, simple 2A3 design. Having looked at Arnold Cruz beautiful simple design I've gor two questions regarding the curcuit (and this is probably where it becomes obvious that I'm not an electrical engineer...)
The cathode resistor in the design says 880 Ohms, but basd on previous post by Arnold I'm thinking about going for 820 instead. What confuses me is that Arnold's circuit says that the potentiometer shall be 50 Ohm, AND 50W - Does it really have to be 50 W? I've studied some other similar designs and no one says anything abut the ability to handle 50w...(for example the JE Labs one)
Am i totally at a loss here going for 820 or what's your recomendadyions? I'll most likely use Hammond 1627 and not the 125 ESE as OPT.
Cheers,
Fredrik
The need for a simple amp has become to much for me, so I decided to go for the JE Labs, simple 2A3 design. Having looked at Arnold Cruz beautiful simple design I've gor two questions regarding the curcuit (and this is probably where it becomes obvious that I'm not an electrical engineer...)
The cathode resistor in the design says 880 Ohms, but basd on previous post by Arnold I'm thinking about going for 820 instead. What confuses me is that Arnold's circuit says that the potentiometer shall be 50 Ohm, AND 50W - Does it really have to be 50 W? I've studied some other similar designs and no one says anything abut the ability to handle 50w...(for example the JE Labs one)
Am i totally at a loss here going for 820 or what's your recomendadyions? I'll most likely use Hammond 1627 and not the 125 ESE as OPT.
Cheers,
Fredrik
Hi Fredrik,
Now I'm put on the spotlight
Is this the amp in the DIY paradise site?
I made the JE Labs SRPP 2A3 as my first tube DIY. I still use it today, but since then converted it to the DX edition (with the 76 tubes, as I like the tone of the 76).
The easy part first-
I used a TDO 50W wirewound hum balance pots because my friend is selling them cheap... cheaper than the Clarostat (10W) that RS Components sells. I bought the Clarostat for my 45 amplifier, but ended up not using them because the center tapped filament supply of my 45 is silent enough.
That TDO hum balance I have removed from my 2A3 and is now on my 300B. Since my 2A3 has center tapped filament supply from the small Hammond filament transformers, I found it not needed, again.
No, you don't need 50W for that, but because of the prior reason (cheap) and having over-spec'ed parts, give me peace of mind.
The 2A3 cathode resistor determines the bias of the output tube and current draw.
With a B+ of 300V, the 880R cathode resistor will give you a bias of 48V at 55mA. With 820R cathode resistor, you'll get 48V at 58mA. Pick your poison, as they say.
I hope I did not confuse you even further
ps.
I use MagneQuest DS-025 on my 2A3 and I love 'em
Now I'm put on the spotlight
Is this the amp in the DIY paradise site?
I made the JE Labs SRPP 2A3 as my first tube DIY. I still use it today, but since then converted it to the DX edition (with the 76 tubes, as I like the tone of the 76).
The easy part first-
I used a TDO 50W wirewound hum balance pots because my friend is selling them cheap... cheaper than the Clarostat (10W) that RS Components sells. I bought the Clarostat for my 45 amplifier, but ended up not using them because the center tapped filament supply of my 45 is silent enough.
That TDO hum balance I have removed from my 2A3 and is now on my 300B. Since my 2A3 has center tapped filament supply from the small Hammond filament transformers, I found it not needed, again.
No, you don't need 50W for that, but because of the prior reason (cheap) and having over-spec'ed parts, give me peace of mind.
The 2A3 cathode resistor determines the bias of the output tube and current draw.
With a B+ of 300V, the 880R cathode resistor will give you a bias of 48V at 55mA. With 820R cathode resistor, you'll get 48V at 58mA. Pick your poison, as they say.
I hope I did not confuse you even further
ps.
I use MagneQuest DS-025 on my 2A3 and I love 'em
Hello Arnold,
Thank you for a quick reply. Yes - I'm looking to build the amp based on your circuit from DIY Paradise. I find your PS solution more appealing than the one on angela.com.
I believe I will go for the 820 resistor, mainly because it's available at my local store - for hum balance I'll drop down to 10W, or could I possibly go lower?
Also, I'm looking to change the 166L2 transformers to one 266M5 which has two secondaries of 2,5V, 3A - I guess that will not pose a problem (?).
Thanks again,
As soon as I have finnished the amp I'll post some pictures!
Cheers,
Fredrik
Thank you for a quick reply. Yes - I'm looking to build the amp based on your circuit from DIY Paradise. I find your PS solution more appealing than the one on angela.com.
I believe I will go for the 820 resistor, mainly because it's available at my local store - for hum balance I'll drop down to 10W, or could I possibly go lower?
Also, I'm looking to change the 166L2 transformers to one 266M5 which has two secondaries of 2,5V, 3A - I guess that will not pose a problem (?).
Thanks again,
As soon as I have finnished the amp I'll post some pictures!
Cheers,
Fredrik
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.